2 Methods of Cryopreservation in Fungi

2
Methods of Cryopreservation
in Fungi
Ladislav Homolka
Abstract
Traditional method of the routine subculturing by transfer of fungal
cultures from staled to fresh media is not a very practical means of storing
large numbers of fungal cultures. It is time-consuming, prone to contamination, and does not prevent genetic and physiological changes. At present, besides freeze-drying (lyophilization), cryopreservation seems to be
the best preservation technique available for fungi.
Keywords
Cryopreservation • Fungi collections • Liquid nitrogen • Perlite
• Subculturing • Fungi storage
Introduction
Serious mycological (and generally biological)
work requires a reliable source of cultures (i.e.,
well-defined and taxonomically determined starting material), which is ensured by its safe longterm storage. This implies the fundamental and
growing importance of culture collections not
only for preservation of the endangered genofond (and consequently the biodiversity), but
also as a principal source of material for biotechnological processes, research, and teaching. The
L. Homolka (*)
Department of Ecology of Microorganisms,
Institute of Microbiology, Academy
of Sciences of the Czech Republic, Vídeňská 1083,
Prague 4 142 20, Czech Republic
e-mail: homolka@biomed.cas.cz
first and most important problem to be solved is
the long-term maintenance of this material.
Collections of fungi were originally kept by
serial transfers from staled to fresh media. This
routine subculturing is not a very practical method
for storing large numbers of fungal cultures. It is
time-consuming, prone to contamination, and
does not prevent genetic and physiological
changes (degeneration, aging) during long-term
and frequent subculturing [1]. Over the years,
various storage methods have been developed in
order to eliminate these disadvantages. Their
common feature is at least partial suppression of
growth and metabolism of the cultures. Among
them, keeping fungal cultures in sterile water
[2–8] was surprisingly efficient (especially with
lower fungi, but also with some basidiomycetes)
and experiences its revival. In some fungi, preservation under a layer of mineral oil, in silica
gel, soil, or sand [9–13] was successful. These
V.K. Gupta et al. (eds.), Laboratory Protocols in Fungal Biology: Current Methods in Fungal Biology,
Fungal Biology, DOI 10.1007/978-1-4614-2356-0_2, © Springer Science+Business Media, LLC 2013
9
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methods enabled a reduction in fungal growth
and extended the time intervals between transfers
to fresh media. Nevertheless, the method of serial
subculturing is still used in collections with limited financial support or in the majority of other
collections as a backup preservation method.
Searching for improved or new methods
resulted through different intermediate steps in
the introduction of lyophilization and cryopreservation of fungal cultures [14]. Application of the
most extended method of culture preservation—
freeze-drying (lyophilization), tested for sporulating fungi as early as 1945 [15]—is rather
limited in the case of basidiomycetes and other
fungi nonsporulating in vitro [16, 17]. Most
attempts to revitalize dehydrated hyphae of fungi
have failed, except for some successes [18–20];
nevertheless, the real absence of spores must be
always carefully checked. Despite this, several
attempts have recently been made using modified
protocols [21–23] and the growing interest in this
technique can be seen at present. An important
role played in the whole process, besides the
freeze-drying, is also the freezing rate and the
lyoprotectant used [24, 25]. Freeze-drying of
fungi has several important advantages over all
other maintenance methods. Cultures can be
stored easily in dense packing without any special requirements and need not be revived on agar
slants prior to dispatch. The product is light, inactive, and dry, enabling easy distribution by mail.
The new or modified methods have been frequently used and evaluated [5, 6, 10, 26–31], but
they are not generally applicable. Often a specific
preservation protocol is necessary even for individual strains of the same species. At present,
besides freeze-drying (lyophilization), cryopreservation seems to be the best preservation
technique available for filamentous fungi [14,
26, 32]. A very comprehensive and detailed
overview of the methods and results of the cryopreservation in microorganisms was published
by Hubálek [33].
Cryogenic technique for long-term storage of
large numbers of fungal species was introduced
to ATCC in 1960 and the results have been very
satisfactory [34, 35]. The technique was consecutively introduced to many other prominent
L. Homolka
collections, e.g., CAB International Mycological
Institute [1], etc. In certain collections—e.g.,
IFO (Institute for Fermentation Osaka)—
nonsporulating cultures of basidiomycetes are
stored by cryopreservation at −80°C in electric
freezers [36].
Mycelium and/or spore suspensions with or
without a cryoprotectant in sealed glass ampoules
were originally used for cryopreservation of
filamentous fungi. Later, glass ampoules were
replaced with safer polypropylene cryovials
and/or straws. Agar blocks immersed in an
appropriate cryoprotectant were originally used
as carriers of fungal mycelium for the cryopreservation process [37]. A useful straw technique with agar miniblocks for the preservation
of fungi in liquid nitrogen was developed by
Elliott in 1976 [38] and improved by Stalpers
et al. [39]. Another technique using straws in
cryotubes without a cryoprotectant solution was
described by Hoffmann [40]. A modified
Hoffmann’s technique was compared with the
original agar block one in our paper [29].
Commercial preservation systems with polystyrene beads as carriers were used for cyopreservation of conidia of enthomopathogenic fungi
[41] and of sporulating Aspergillus fumigatus
cultures at −80°C [42]. Porous ceramic beads
were employed for cryopreservation of several
sporulating fungal cultures and for a
Saccharomyces cerevisiae culture at −70°C by
Palágyi et al. [43]. It is symptomatic that these
techniques have not been used for nonsporulating filamentous fungi. In this context it should
be mentioned that as early as 1978 Feltham et al.
described a method of preservation of bacteria
on glass beads at −76°C [44]. Some reports [45–
47] indicate that cryopreservation at −80°C is
suitable for many fungal cultures, including
basidiomycetes. Nevertheless, Leeson et al. [48]
state that to completely stabilize frozen cultures,
the temperature must be sufficiently reduced to
both minimize metabolism and prevent ice crystal formation, which can cause physical damage
during storage. The temperature limit securing
prevention of formation of such ice crystals is
−139°C. This is why at present many culture
collections start to keep their cultures at −150°C
2
Methods of Cryopreservation in Fungi
in ultralow-temperature electric freezers, which
are sometimes equipped with liquid nitrogen
supply.
The cryopreservation process includes freezing and thawing and the protocol of these procedures plays an important role [49, 50]. In
principle, there are two kinds of freezing protocols: a slow (controlled) one and a fast (uncontrolled) one, which both have been used for
cryopreservation of fungi [51, 52]. Generally,
too low freezing rates cause excessive dehydration and concentration of the solution leading to
cell damage; on the contrary, too fast freezing
leads to insufficient dehydration and formation
of abundant ice crystals with lethal consequences.
Nevertheless, different fungal cultures exhibit
different sensitivities to freezing conditions and
to the presence and concentration of cryoprotectants. A freezing rate of 1°C per minute is usually used for cryopreservation of fungi; in the
author’s experiments with sensitive mutant
strains of Agaricus bisporus, the freezing rate
0.5°C per minute was successfully used.1 Lately,
cryomicroscopic methods have been used to
study the process of freezing and thawing of fungal cultures [14, 24, 53]. Successful cryopreservation depends on the cryoprotectant used [54].
At present, dimethylsulfoxid and glycerol are the
most widespread [55]. The method of cryogenic
culture maintenance seems to be mostly successful also in nonsporulating cultures [14, 32, 56,
57]. An overview study on the influence of the
cryopreservation process on survival of taxonomically very broad spectrum of fungi published by Smith [14] showed that there was no
obvious link between taxonomic grouping and
the response of the fungi to freezing and thawing. This was confirmed in our study [31]. Similar
studies were carried out also in edible mushrooms Lentinus edodes [58] and the genus
Pleurotus [59]. Davell and Flygh [60] showed
that even an ectomycorrhizal fungus
Cantharellus cibarius can be successfully cryopreserved when a sufficient number of cryoprotocols is tested. Cryopreservation of spores of
1
Homolka, unpublished results.
11
vesicular–arbuscular mycorrhizal fungi was
described by Douds and Schenck [61].
Beyond survival, another principal requirement for the successful preservation of fungal
strains is maintenance of their genetic and physiological features, such as growth, morphology,
and metabolite production. In our experiments
with some white-rot basidiomycetes, no negative
effect of cryopreservation or the used cryoprotective on production of ligninolytic enzymes was
found [62]. The complete revival of cryopreserved cultures (evaluated mostly by measuring
the colony diameter) is generally still uncertain.
The survival rate varies between 60 and 100%
[21, 58, 59, 63, 64]. Only a few studies of the
genetic stability of cryopreserved fungi have been
performed. Singh et al. [47] confirmed the genetic
stability of 11 cryogenically preserved edible
mushroom strains by comparing random
amplified polymorphic DNA (RAPD) and internal transcribed spacer (ITS) profiles. Using polymerase chain reaction (PCR) fingerprinting, Ryan
et al. [65] checked the genetic stability of several
isolates of Fusarium oxysporum and Metarhizium
anisopliae. Other studies include confirmation of
the genetic stability of Uncinula necator conidia
after storage at −80°C [45] and investigation of
the influence of mid-term cryopreservation at
−80°C on 15 isolates of 10 basidiomycete species, for which the DNA fingerprint patterns were
unchanged [66]. All of these reports were solely
based on fingerprinting methods, which are not
suitable for the detection of minute yet important
changes in the genome, such as point or short
indel mutations. Rather, sequencing approaches
are required to successfully detect these mutations. This approach was used in our recent study
[67]. Considering the above data, there is a continuous need for developing, improving, optimizing, and combining of preservation procedures,
because the present methods are not applicable to
all fungal cultures. Although many of these fungi
can be grown in pure cultures on solid media,
their growth is often attenuated and their morphology and other characteristics changed, which
can result in their complete loss. The number of
characteristics evaluating the success of preservation should be increased.
L. Homolka
12
As mentioned previously, cryopreservation,
namely in liquid nitrogen, seems to be the most
reliable, safe, and prospective method of a
long-term maintenance of most fungal species,
especially those not amenable to freeze-drying.
It is probably the only storage technique that
can ensure genomic and phenotypic stability.
But not even the aforementioned cryopreservation method is applicable to preservation of all
fungal cultures in the present form. According
to the literature as well as the author’s personal
experience, especially the maintenance of
basidiomycetes is challenging. Many of these
fungi do not form asexual spores, their dominant life form, the vegetative mycelium, is sensitive to environmental conditions and therefore
not amenable to freeze-drying.
To address these issues, a method of cryopreservation using perlite as a carrier for fungal
mycelia was developed in the author’s laboratory
(perlite protocol or PP) [28] and then successfully verified for 442 basidiomycete strains [30].
Perlite is a unique aluminosilicate volcanic mineral that retains substantial amounts of water that
can be released when needed—a feature that
seems to have a dominant effect on cryopreservation success. The PP can be used for cryopreservation of taxonomically different groups of fungi,
including yeasts [31], and works relatively well
for fungi that cannot survive other routine preservation procedures. Expanded perlite was used as
a solid support in solid-state fermentations [68];
otherwise it is used in many applications, particularly in the construction, horticulture, and other
various industrial fields. It is recommended as an
efficient purifying agent and as a carrier for pesticides, feed concentrates, herbicides, and other
similar applications.
Materials
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23.
Perlite Protocol (PP)
The protocol is suitable for maintaining a broad
spectrum of fungal cultures of different origin. It
was verified in several culture collections (e.g.,
in Finland, the Netherlands, USA, Czech
Republic, etc.) with great success and now it is
routinely used there.
2
Distilled water.
Agar Difco.
Glycerol p.a. Sigma.
Isopropyl
alcohol
100%
Sigma
(alternatively).
Agricultural-grade
perlite
(Agroperlit,
GrowMarket s.r.o., Prague, Czech Republic,
http://www.growmarket.cz/produkt/agroperlit-8l)—particles 1–2 mm.
Dried wort extract Sladovit, Malthouse
Bruntál, Bruntál, Czech Republic, diluted to
a density of 4° Balling scale with distilled
water (further wort in the text); or preferably
MYA medium: Malt-extract Difco 25 g,
Yeast-extract Difco 2 g, Glycerol p.a., Serva
50 g, distilled water ad 1,000 mL. pH adjusted
to 6.5 with 1 M KOH solution.
Ethanol (70%).
Liquid nitrogen (LN).
Nunc CryoTube Vials 1.8 mL screw-capped
(Nalgene/Nunc, Rochester, USA).
Cork borer.
Lancet and small spoon.
Rule for measuring of colony diameter.
Water bath.
Balling
hydrometer
(saccharometer)
(alternatively).
Hot-air sterilizer.
Autoclave.
Refrigerator (about 4°C).
Deep-freezer (−80°C).
Microscope equipped with phase contrast.
Laminar flow box.
Thermostat (incubator) for 24°C.
Container for storing samples in liquid nitrogen (e.g., HARSCO TW-5K container,
Harsco, Camp Hill, USA).
Programmable freezer for controlled freezing of cryovials with mycelia (e.g., IceCube
1800 freezer, SY-LAB Geraete GmbH.,
Neupurkersdorf, Austria); or alternatively
Cryo 1°C Freezing Container “Mr. Frosty”
(Nalgene Labware)2.
All chemicals and devices named can be replaced with
other ones produced by other renowned companies.
2
Methods of Cryopreservation in Fungi
Methods
The methods given below describe general
procedures for cryopreservation of fungi using
the perlite protocol (PP).
Strains (Cultures)
The starting cultures are kept on wort agar slants
or dishes (wort 4° Balling, 1.5% agar Difco) at
4°C or other media suitable for the growth of the
strains destined for the procedure (e.g., MYA
medium with 1.5% agar, etc.) and transferred to
the fresh medium every 6 months.
1. Prepare an agar medium, sterilize it in an autoclave (121°C, 20 min.), pour it into sterile
plastic Petri dishes (diameter 100 mm, 30 mL
per dish), and let it cool down in a laminar
flow box.
2. In a laminar flow box cut out an agar plug
(6 mm diameter) from the actively growing
part of a colony on a Petri dish with a cork
borer, place it on a Petri dish with fresh medium
using a sterile lancet and then let the dish incubate for 14 days at 24°C. Then put the dish(es)
into a refrigerator and keep it at about 4°C.
13
on a Petri dish using a cork borer, place it
using a sterile lancet on the surface of perlite
in the cryovial, close the vial firmly and let it
incubate for 14 days at 24°C.
2. Freeze the cryovials with perlite overgrown by
mycelium in a programmable freezer (or alternatively in a “Mr. Frosty” container in a deepfreezer) to −70°C at a freezing rate of 1°C per
minute. Then place them in LN in a container.
3. Take the stored frozen cultures in cryovials out
of the LN container, transfer them to a water
bath (37°C), and leave them there until the ice
is completely thawed (thawing—reactivation
of cultures). Prior to opening, disinfect the surface of cryovials with 70% ethanol.
Viability Test
1. After thawing, separate at least partially the
perlite particles overgrown with mycelium by
shaking, the content of the cryovials (two parallels of each strain) divide into three approximately equal aliquots each and these plate onto
wort (or an other) agar medium in Petri dishes
(diameter 100 mm) using a small sterile spoon.
2. Incubate the cultures in Petri dishes at 24°C
for 14 days. Strains exhibiting survival of at
least four out of six separate aliquots are considered viable.
Culture Preparation and Freezing–
Thawing Protocols
Fungal cultures are grown directly in firmly
closed sterile plastic cryovials (1.8 mL) with
200 mg of perlite (Agroperlit, agricultural grade)
moistened with 1 mL of wort (4° Balling) or other
medium (e.g., MYA) enriched with 5% glycerol
as a cryoprotectant. For sterilization of the cork
borer, lancet, and small spoon use a hot-air sterilizer (150°C, 30 min.).
1. Distribute perlite into cryovials (200 mg per
vial), flood it with 1.8 mL of the medium
enriched with 5% of glycerol, and sterilize
vials in autoclave (121°C, 20 min.). In a laminar flow box cut out an agar plug (6 mm diameter) from the actively growing part of a colony
Growth Estimation and Morphological
Analysis
1. Growth of cultures measure as a mean diameter increase of a growth-covered zone (in mm)
during a 14-day incubation at 24°C on the
respective agar medium in Petri dishes (diameter 100 mm) inoculated with perlite aliquots
from cryovials before freezing and after reactivation. Measure six zones (three aliquots
from two cryovials) for each strain. The first
occurrence of growth varies between frozen
cultures, with some strains showing signs of
re-growth within 2 days but most strains reactivating within 7 days after plating.
14
2. Use the same procedure except for freezing
and thawing for growth measurement of the
control.
3. Carry out the morphological analysis on control cultures and on those arising from the
viability tests. Check the selected macroscopic
features (colony color, reverse color, texture
of the mycelium) and microscopic features
(hyphal branching, presence/absence of clamp
connections, presence/absence of hyphal vacuolization, etc.) using a microscope.
4. If possible, estimate also other characteristics
of the resulting cultures (e.g., enzyme or
metabolite production, etc.) according to your
consideration.
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